Optical time domain reflectometer and method

ABSTRACT

An optical time domain reflectometer has an acoustic-optic modulator (30) for generating three different wavelengths, optical multipliers 40, 42 for alternating between one of the wavelengths (λ 1 ) and the other two wavelengths (λ 1  +Δλ, λ 1  -Δλ) in response to a signal generator 44, enables the provision of a test signal for transmission on an optical fiber transmission cable.

BACKGROUND OF THE INVENTION

1. Technical Field

This invention relates to an optical time domain reflectomer (OTDR) anda method of fault or break detection by optical line domainreflectometry.

Optical communication systems are increasingly relying on the use ofoptical amplifiers to boost the optical signals so that communication athigh bit rates over long ranges can be achieved. The cable andamplifiers for such systems are often located in inaccessible places, soa method of fault finding remotely from the ends of the system isrequired.

2. Description of the Prior Art

One method for measuring faults in fiber spans which has been usedextensively is the optical time domain reflectometer. This injects anoptical pulse into the fiber and then measures the relative magnitudeand delay of the back scatter from the pulse as it propagates throughthe fiber. Generally optical amplifiers used in commercial applicationsinclude an optical isolator which prevents the backscattered light frompassing back through the amplifier. The isolator therefore precludes theuse of this type of OTDR on fiber spans employing optical amplifiers.

Several methods have been proposed for fault location on a bidirectionaloptically amplified line e.g. R. Jensen, C. Davidson, D. Wilson and J.Lyons. "Novel Technique for Monitoring Long Haul Undersea OpticalAmplifier Systems", Conference on Optical Fiber Communications 1994,Paper No. ThR3. The most convenient is the inclusion of cross couplersbetween the send and return amplifiers within a repeater as illustratedin FIG. 1. Note: Amplifiers A & C are an amplifier pair in one repeater,amplifiers B & C are an amplifier pair in the next repeater in the line.

Conventional OTDR's using an amplitude modulation pulse stream are notsuitable for use in a system employing optical amplifiers since theamplifiers require a constant mean power with respect to the erbiumfiber time constant. To overcome this it has been proposed by Y.Horiuchi, S. Ryu, K. Mochizuki and H. Wakabayashi, in "Novel CoherentHetrodyne Optical Time Domain Reflectometry for Fault Localization ofOptical Amplifier Submarine Cable Systems". IEEE Photonics TechnologyLetters, Vol 2, No. 4 (April 1990), that the pulses used are in the formof frequency shift keying (FSK) of an optical carrier (as in FSK RADAR).

Such systems suffer from degradation due to polarization effects and thepresent invention seeks to reduce the effect of polarization.

SUMMARY OF THE INVENTION

The present invention seeks to provide an improved optical time domainreflectometer.

According to the invention there is provided an optical time domainreflectometer comprising means for generating three differentwavelengths and means for alternating between one of the wavelengths λ₁and the other two wavelengths in response to a signal generator forproviding a test signal for transmission on an optical fibertransmission cable. By employing a different wavelength for alternatesignal portions, polarization reflection states would be different ineach case so that polarization sensitivity is reduced.

The three different wavelengths may be derived from a common narrowlinewidth laser source by means of one or more optical modulatorsresponsive to an RF signal which is triggered to provide the alternationbetween one of the wavelengths being the narrow linewidth, and the othertwo wavelengths. In one embodiment of the invention, a singleacousto-optical modulator is coupled to the laser source and is providedwith oppositely disposed modulating inputs driven alternately from an RFgenerator in response to a pulse signal generator. The modulator mayhave individual outputs for the three wavelengths, which outputs arecombined onto a single line by optical coupling means to provide thetest signal for transmission.

In another embodiment first, second and third acousto-optical modulatorsmay be provided, all optically coupled to the output of the lasersource, the first and second of which modulators are coupled to an RFgenerator which provides a modulating signal, the third of whichmodulators is gated on and off to communicate the laser wavelength to anoutput line, and fourth and fifth acousto-optic modulators coupledbetween the output of first and second acousto-optic modulatorsrespectively to an optical coupler which communicates to the outputline. A complementary code generator is effective to provide a gatingsignal to the third acousto-optic modulator to gate "on" in the absenceof a test signal transmission and "on" and "off" during a test signaltransmission when it provides complementary gating signals to the fourthand fifth acousto-optical modulators to provide the test signal fortransmission.

The reflectomer may comprise an optical receiver for receiving the testsignal after reflection from the optical fiber transmission cable, thereceiver being arranged to hetrodyne the received wavelengths with oneof the wavelengths such that the difference frequency between thatwavelength and the other returning wavelengths are detected; an envelopedetector for the detected difference frequency signals; and a signalprocessor for determining the location of a break or fault in thetransmission cable as a function of the time delay between transmittedand received test signals.

Alternatively, the reflectomer may comprise two optical receivers bothcoupled to receive the test signal after reflection from the opticalfiber transmission cable, wherein one receiver is arranged to heterodynethe received wavelengths with a first one of the other two wavelengthsand is routed via a low pass filter and envelope detector to a signalprocessor to provide an input indicative of the receipt of the first oneof the wavelengths and the other receiver is arranged to heterodyne thereceived wavelengths with a second one of the other two wavelengths andis routed via another low pass filter and envelope detector to thesignal processor such that the signal processor can distinguish betweenthe received first and second wavelengths thereby to permit transmissionand detection of a bipolar test signal. The test signal transmission mayconstitute a complimentary code e.g. Golay code and the signal processormay be adapted to detect that code.

In one advantageous embodiment the other two wavelengths areequidistantly displaced in frequency one above and one below thefrequency of the one of the wavelengths.

According to another aspect of the invention there is provided a methodof fault detection in an optical fiber transmission line comprising thesteps of: generating three different wavelengths; transmitting anoutgoing signal which comprises a first one of the wavelengths in theabsence of a test signal and a digital test signal which alternatesbetween the first wavelength and a second and third wavelength toprovide the test signal; receiving and detecting the test signal afterreflection from a fault or break on the line; and determining thelocation of the fault or break as a function of the time delay betweentransmission and receipt of the test signal.

The invention will be fully understood when reference is made to thefollowing detailed description taken in conjunction with theaccompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

In order that the inventions and its various other preferred featuresmay be understood more easily, some embodiments thereof will now bedescribed by way of example only with reference to the drawing, inwhich:

FIG. 1 illustrates schematically a conventional system which permitsoptical time domain reflectometers to be employed in an opticallyamplified system for detection of breaks.

FIG. 2 is a block schematic diagram of an OTDR constructed in accordancewith the invention and suitable for use in the system of FIG. 1.

FIG. 3 is a waveform amplitude/time diagram to illustrate the signalsgenerated by the OTDR of FIG. 2. and

FIG. 4 is a block schematic diagram of an alternative OTDR constructedin accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1, an outgoing optical fiber line 10 for transmission of traffichas a pair of amplifiers A & B and is illustrated to have a break 12. Areturn optical fiber line 14 for receipt of traffic has a pair ofoptical amplifiers D & C. An OTDR transmission (not shown) is coupled toinput 16 so as to launch a pulse onto the line 10. The pulse isreflected or backscattered from the break 12 and is coupled via opticalcouplers 18, line 20 and optical coupler 22 across to the return line 14and is amplified by the amplifier C before passing out of the output 24of the return line to an OTDR receiver. The present invention is useablein such a system.

Referring now to FIG. 2, there is shown an acousto-optic modulator 30for example a Bragg cell which is employed to generate three wavelengthsfrom a single narrow linewidth laser source 32, e.g. a distributedfeedback laser or an external cavity laser. A suitable laser wavelengthis 1550 nm. An RF generator 34 (also known as an oscillator) having afrequency of e.g. 50 MHz is coupled to oppositely disposed modulatorinputs 36, 38 via individual optical multiplier or optical switches 40,42 (also known as a mixer). The multipliers 40, 42 are each coupled to adifferent outputs of a pulse generator 44 which outputs are arrangedboth to provide a logic 0 out in a quiescent state and to providecomplementary pulses when in operation to generate a test signal. In thequiescent state a logic 0 applied to each of the multipliers preventspassage of the RF drive frequency to the inputs 36 and 38 of themodulator 30. When the pulse generator 44 is in operation it providescomplementary switching pulses to the multipliers 40, 42 so that the RFdrive is alternately switched between the inputs 36 and 38 in responseto generator pulses such that the modulator 30 produces the waveformillustrated in FIG. 3 where λ₁ is the wavelength of the laser 32 and Δλis the wavelength equivalent of the 50 MHz RF generator 34. The pulsesλ₁ +Δλ and λ₁ -Δλ may or may not be the same amplitude as the signal λ₁.The acousto-optic modulator 30 has three outputs one for λ₁ one for λ₁/+Δλ and one for λ₁ /-Δλ. The outputs for wavelengths λ₁ +Δλ and λ₁ -Δλare each coupled via optical couplers 46, 48 to an output line 50 forthe wavelength λ1 and the output line 50 is connected via an amplifier52 to a transmission system under test. A return line 54 from anamplifier 56 is coupled to the input of an optical receiver 58 andreceives the reflected OTDR signals. The output of the laser 32 atwavelength λ₁ is coupled via optical coupler 60, optical fiber 62 andoptical coupler 64 to the return line 54 and into the receiver 58. Thereceiver 58 heterodynes λ₁ with the returning signal on line 54 andproduces a common difference frequency of wavelength Δλ (50MHz) fromboth wavelengths λ₁ +Δλ and λ₁ -Δλ and this permits the generation of acommon frequency for each test pulse. This is then passed through abandcase filter and envelope detector circuit 66 to a signal process 59.The signal processor 59 also receives pulses from the pulse generator 44and compares the time delay between transmission and reception of testpulses and determines the location of the break as a function of thetime delay.

The arrangement of FIG. 2 is advantageous in that a single laser isemployed to generate three wavelengths. The use of three wavelengths hasa beneficial effect in reducing the effect of polarization sensitivitywhich results in reduction of dynamic range of the systems. However, thearrangement does not permit distinction between the two return pulsewavelengths, as they both have the same frequency after heterodyning,and this prevents the use of bipolar complementary coding.

FIG. 4 illustrates an alternative OTDR which permits distinction of abipolar complementary coded test signal. In this arrangement a narrowlinewidth laser source 70 of wavelength λ₁ has an output line 72. First,second and third acousto-optical modulators 74, 76 and 78 respectivelyare optically coupled to the line 72. The first and second modulatorsare coupled to the line 72 via optical couplers 80, 82. The first andsecond modulators 74, 76 are each coupled to an RF generator 77 of e.g.50 MHz so as to provide a wavelength at their outputs of λ₁ +Δλ & λ₁ -Δλrespectively. The third modulator 78 has an output to a line 84 to asystem under test and has a gating input 86 which receives an outputfrom a complementary code generator 88 which is effective to gate themodulator 78 on and off to communicate the laser wavelength λ₁ to theoutput line 84. Fourth and fifth acousto-optic modulators 90, 92 arecoupled between the output of first and third acousto-optic modulators74, 76 respectively and to an optical coupler 94, 96 which communicatesto the output line 84. The complimentary code generator 88 has twofurther outputs coupled one to each of the modulators 90, 92 via gatinginputs 98, 100. The complementary code generator 88 is effective toprovide a gating signal to the third modulator 78 to gate on in theabsence of a test signal transmission and on and off during a testsignal transmission when it provides complementary gating signals to thefourth and fifth modulators 90, 92 so as to provide on the output line84 a signal which alternates between wavelength λ₁ and λ₁ +Δλ and λ₁-Δλ.

The receiving path 102 from the line is split via an optical coupler 104into two paths one to each of two optical receivers 106, 108 so thatthey receive the test signals after reflection from the fault in theoptical fiber transmission cables. The input to one of the receivers 106is coupled to the output of modulator 76 and receives the additionalwavelength λ₁ +Δλ whilst the input to the receiver 108 is coupled to theoutput of modulator 74 and receives the additional wavelength λ₁ +Δλ.These additional wavelengths heterodyned with the received wavelengthsand are routed via a low pass filter and envelop detector 110 or 112 toa signal processor 114. Accordingly the signal processor 114 candistinguish between the received first and second wavelengths. Thispermits the transmission and detection of a bipolar test signal whichcan be transmitted in complementary code pairs such as a Golay code.Employment of Complementary coding results in increased sensitivity ofdetection and hence dynamic range.

The preferred embodiment described above admirably achieves the objectsof the invention. However, it will be appreciated that departures can bemade by those skilled in the art without departing from the spirit andscope of the invention which is limited only by the following claims.

What is claimed is:
 1. An optical time domain reflectometercomprising:means for generating three different wavelengths (λ₁, λ₁ +Δλ,λ₁ -Δλ); means for alternating between one of the wavelengths (λ₁) andthe other two wavelengths (λ₁ +Δλ, λ₁ -Δλ) in response to a pulsegenerator signal from a signal generator, for providing a test signalfor transmission on an optical fiber transmission cable; and means forheterodyning the test signal after reflecting from the opticaltransmission cable with one of the three different wavelengths (λ₁, λ₁+Δλ, λ₁ -Δλ).
 2. An optical time domain reflectometer as recited inclaim 1, wherein the three different wavelengths are derived from acommon narrow linewidth laser source by means of one or more opticalmodulators responsive to an RF signal which is triggered to provide thealternation between one of the wavelengths (λ₁) that is a narrowlinewidth, and the other two wavelengths (λ₁ +Δλ, λ₁ -Δλ).
 3. An opticaltime domain reflectometer as recited in claim 2, wherein a singleacousto-optical modulator is coupled to the common narrow linewidthlaser source and is provided with oppositely disposed modulating inputsdriven alternately from an RF generator in response to the signalgenerator.
 4. An optical time domain reflectometer as recited in claim3, wherein the single-acoustic optical modulator has individual outputsfor the three different wavelengths (λ₁, λ₁ +Δλ, λ₁ -Δλ), which outputsare combined onto a single line by optical coupling means to provide thetest signal for transmission.
 5. An optical time domain reflectometer asrecited in claim 2, further comprising:first, second and thirdacousto-optical modulators, all optically coupled to an output of thecommon narrow linewidth laser source, the first and secondacoustic-optical modulators are coupled to an RF generator whichprovides a modulating signal, the third acoustic-optical modulator isgated on and off to communicate a laser wavelength to an output line;and fourth and fifth acousto-optic modulators coupled between an outputof the first and second acoustic-optic modulators respectively to anoptical coupler which communicates to the output line; wherein acomplementary code generator is effective to provide a gating signal tothe third acousto-optic modulator to gate on in the absence of a testsignal transmission and to gate on and off during the test signaltransmission when to provide complementary gating signals to the fourthand fifth acousto-optical modulators to transmit the test signaltransmission.
 6. An optical time domain reflectometer as recited inclaim 2, wherein the single-acoustic optical modulator has individualoutputs for the three different wavelengths, which outputs are combinedonto a single line by optical coupling means to provide the test signalfor transmission.
 7. An optical time domain reflectometer as recited inclaim 1, comprising:an optical receiver coupled with an envelopedetector, the optical receiver being arranged for receiving the testsignal after reflection from the optical fiber transmission cable,wherein the optical receiver is arranged to heterodyne the receivedwavelengths with said one of the wavelengths such that the differencefrequency between that wavelength and the other returning wavelengthsare coupled to the envelope detector for detecting difference frequencysignals; and a signal processor for determining the location of a breakor fault in the optical fiber transmission cable as a function of thetime delay between transmitted and received test signals.
 8. An opticaltime domain reflectometer as recited in claim 7, wherein the test signaltransmission constitutes a complimentary code and the signal processormay be adapted to detect the complimentary code.
 9. An optical timedomain reflectometer as recited in claim 8, wherein said other twowavelengths are adequately displaced in frequency one above and onebelow the frequency of said one of the three different wavelengths (λ₁,λ₁ +Δλ, λ₁ -Δλ).
 10. An optical time domain reflectometer as recited inclaim 1, wherein said other two wavelengths are adequately displaced infrequency one above and one below the frequency of said one of the threedifferent wavelengths.
 11. A method of fault detection in an opticalfiber transmission line comprising the steps of:generating threedifferent wavelengths, transmitting an outgoing signal which comprises afirst one of the three different wavelengths in the absence of a testsignal and a digital test signal which alternates between the first oneof the three different wavelengths and a second and third wavelength toprovide the test signal, receiving and detecting the test signal afterreflection from a fault or break on the optical transmission line,heterodyning a received test signal with one of the three differentwavelengths, and determining the location of the fault or break as afunction of the time delay between transmission and receipt of the testsignal.
 12. An optical time domain reflectometer comprising:means forgenerating three different wavelengths (λ₁, λ₁ +Δλ, λ₁ -Δλ); means foralternating between one of the wavelengths λ₁ and the other twowavelengths (λ₁ +Δλ, λ₁ -Δλ) in response to a pulse generator signalfrom a signal generator, for providing a test signal for transmission onan optical fiber transmission cable; and two optical receivers bothcoupled to receive the test signal after reflection from the opticalfiber transmission cable, wherein one receiver is arranged to heterodynethe received wavelengths with a first one of said other two wavelengths(λ₁ -Δλ) and is routed via a low pass filter and envelope detector to asignal processor to provide an input indicative of the receipt of thefirst one of the wavelengths and the other receiver is arranged toheterodyne the received wavelengths with a second one of said other twowavelengths (λ₁ +Δλ) and is routed via another low pass filter andenvelope detector to the signal processor such that the signal processorcan distinguish between the received first and second wavelengthsthereby to permit transmission and detection of a bipolar test signal.